Method for gas-liquid disentrainment operations



3 ,4 10,05 7 ERATIOILJS B. J. LERNER Nov. 12, 1968 METHOD FOR GASLIQUIDDISENTRAINMENT OP 5 Sheets-Sheet l Filed Sept.

H mm T R N E EL WJ I D R A N R E B ATTORNEY B. J. LERNER Nov. 12, 1968METHOD FOR GAS-LIQUID DISENTRAINMENT OPERATIONS 5 Sheets-Sheet 2 FiledSept. 28, 1966 INVENTOR. BERNARD J. LERNER ATTORNEY Nov. 12, 1968 B. J.LERNER 3,410,057

METHOD FOR GAS-LIQUID DISENTRAINMENT OPERATIONS Filed Sept. 28, 1966 5Sheets-Sheet (5 OVERHEAD REFLUX FEED T. r

BOTTOMS PRODUCT INVENTOR.

BERNARD J. LERNER BY W ATTORNEY.

Nov. 12, 1968 J. LERNER 3,410,051 METHOD FOR GAS-LIQUID DISENTRAINMENTOPERATIONS 5 Sheets-Sheet 4 Filed Sept. 28, 1966 v2" POROUS CYLINDERS lo 0 o O o 4 2 l m 00 o 1/2 RASCHIG RINGS 1/2" POROUS CYLINDERS was 4 286 2 2000 ELOCITY,

2000 SUPERFICIAL GAS MASS VELOCITY,

SUPERFICIAL GAS MASS V LBS./HR./FT. LBS./HR./FT. F/g. /5 F IN VEN TOR.BERNARD J. LERNER SUPERFICIAL GAS MASS VELOCITY,

LBS /HR./FT.

ATTORNEY B. J. LERNER Nov. 12, 1968 METHOD FOR GAS-LIQUID DISENTRAINMENTOPERATIONS 5 Sheets-Sheet 5 Filed Sept. 28, 1956 O O O 0 0236/3 Fu\O IFm a cmzzmo km PPI COUNT OF PELLET INVENTOR. BERNARD J. LERNER ATTORNEY.

United States Patent ABSTRACT OF THE DISCLOSURE This invention relatesto the use of :a new packing having high efliciency for gas-liquiddisentrainment operations, i.e., the removal of entrained liquiddroplets from a moving gas stream. The new packing comprises randomlydisposed, porous but noncapillary discrete bodies of relatively smallsize made up of interconecting cells of varying size formed by a3-dimensional network of interconnected strands and communicating withone another through pores of varying size, the average pore size beingcontrolled within the limits of from about 5 to 50 pores per linealinch. A prime advantage of the new packing over prior disentrainmentpackings is its high resistance to flooding .at high liquid loadings.

This applications is a continuation-in-part of my copending application,Ser. No. 336,802, filed Jan. 9, 1964, for Packing Element for Gas-LiquidContacting Operations, now abandoned.

A wide variety of packing materials is employed for improving theefiiciency of disentrainment between gas and liquid in connection withoperations such :as distillation, gas scrubbing operations where a gasor vapor component is absorbed by a liquid absorbent, or strippingoperations where a gas or vapor is stripped out of a liquid by astripping gas. One of the most common types of packing for such useconsists of small shapes such as Raschig rings, Berl saddles, and thelike which are dumped into the gasliquid disentrainment space in randomfashion to provide a large solid surface impingement area for the dropsin the carrier stream passing through the voids between the individualpacking pieces. Such solid shapes have the disadvantage that the bed ofpacking is relatively heavy, requiring relatively strong structuralsupports. Such packings also have the disadvantage that they haverelatively high flow resistance and are prone to flooding at moderate oreven relatively low rates of gas and liquid flow (flooding being thepoint at which the gas phase becomes discontinuous, the gas pressuredrop becomes unstable and the bed tends to fill with liquid).

Many types of packings in the form of screens or woven or fibrous matshave :also been suggested. This type of packing has the disadvantagethat the screens or mats must be specially installed, often by tediousprocedures, in contrast to the particulate form of packing which may bedumped at random to form a bed as in the case of R-aschig rings, Berlsaddles or the like. Fibrous mats or pads used as disentrainment devicesare normally operated with a layer of liquid at the bottom of the pad,with the gas bubbling up through the liquid layer. (Flooding in the caseof a pad disentrainer refers to the reentrainment of liquid from thisstanding liquid layer. Such liquid layer thickens with an increase ingas rate.) Because the liquid cannot be easily discharged from thebottom layers of the pad against the frictional resistance of theupflowing gas, particularly in the case of higher gas velocities and/orliquid entrainment loads, the major portion of liquid dischargefrequently takes place down the walls of the vessel. Fur- Patented Nov.12, 1968 ther, it is common practice to enlarge the diameter of thevessel at the point where a pad disentrainer is placed so that the gasvelocity is reduced, and a wall-directed component of gas thrust iscreated to facilitate liquid discharge down the wall. Because a pad isnormally operated with a standing liquid layer, and a discontinuous,bubbling gas flow, it is limited with respect to capacity for highliquid entrainment loads and high gas velocities.

Another type of packing proposed in the past for gasliquid contactingconsists of small hollow shapes, such as hollow cylinders, made offine-mesh screen wherein liquid filming takes place on the surfaces ofthe screen to provide :a high gas-liquid interface for mass transfer.With such packing, the liquid flows on the surfaces of the screen inroughly the same way that liquid flows on the surfaces of conventionalpackings with impervious solid surfaces, and they are accordinglysubject to much the same disadvantages.

In accordance with the present invention, :a new type of packing has nowbeen found providing a unique combination of advantages for gas-liquiddisentrainment operations. It is designed to be randomly dumped into thebed space with no necessity for stacking the packing elements. It isvery light in weight, and can be made from low-cost materials. It ischaracterized by very high free volume (i.e., high pore volume withinthe packing itself) and very low pressure drops even at high rates ofgas and liquid flow. It is highly resistant to flooding at high gas andliquid rates, and thus can be used under conditions in which the gasand/or liquid entrainment load varies over wide ranges.

The new packing of the invention comprises a bed of small, regularlyshaped, porous but noncapillary bodies made up of a multitude ofirregularly shaped, open cells of varying size formed by a multitude ofinterconnected strands, such cells communicating with one anotherthrough irregularly shaped pores of varying size, the average pore sizebeing controlled Within the limits of from about 5 to 50, and preferably10 to 40, pores per lineal inch to insure noncapill-ary, i.e.,nonflooding behavior. The cross-sectional thickness of the porous bodiesmaking up the packing of the invention is large relative to average porediameter. The porous bodies should be sufliciently rigid to resistsubstantial deformation under the load imposed on them during operatingconditions and so shaped that, when dumped into a contact tower inrandom arrangement, a relatively large void volume between theindividual bodies will result, as distinguished from a closepackedarrangement with little or no void volume between the individual bodies.To this end, the individual bodies are shaped so as to insure mostlypoint and line contact, rather than planar contact between the bodies,when dumped randomly in a contact tower, to form a packed bed.

In gas-liquid disentrainment applications, the extreme resistance of thepacking of the invention to flooding even at high liquid loadings givesit outstanding advantages over prior disentrainment devices. The packingof the invention possesses this flood-resistant property by reason of aunique and distinctive liquid channelling be havior not exhibited by apad or any other disentrainment device. That is, such a bed returns thedisentrained liquid through flow channels established entirely withinthe confines of the porous bodies themselves, leaving free for gas flowthe interstitial voids between the porous bodies as well as thoseportions of the porous bodies not utilized by the flowing liquiddischarge.

For a better understanding of the invention, reference is now made tothe accompanying drawings wherein FIG. 1 is a side elevation of oneembodiment of an individual body making up the new packing of theinvention; and

FIG. 2 is a cross-sectional view taken on the line 2-2 of FIG. 1; and

FIG. 3 is an enlarged view of a portion of FIG. 2 showing the details ofthe pore structure of the packing body; and

FIG. 4 is an isometric view showing in enlarged, semiidealized form atypical cell of which the packing body of FIGS. 1 and 2 is composed; and

FIG. 5 is a side elevation of a second embodiment of a body making upthe new packing of the invention; and

FIG. 6 is a cross-sectional view taken on the line 6-6 of FIG. 5; and

FIG. 7 is an enlarged view of a portion of FIG. 6 showing the details ofthe port structure; and

FIG. 8 is an enlarged, isometric, semi-idealized illustration of one ofthe cells making up the packing body of FIGS. 5 and 6; and

FIG. 9 is a view showing a portion of a packed bed made up of packingbodies of the invention, illustrating the type of gas and liquid flowoccurring in the bed; and

FIG. 10 is a view showing a typical application of the new packing ofthe invention, i.e., to provide a disentrainment packing in a tray-typedistillation tower to prevent liquid spray carry-over from one plate toanother; and

FIG. 11 is a vertical cross-sectional view of a portion of the tower ofFIG. 10 showing the tray construction and the manner in which thepacking of the invention is used to provide spray disentrainment; and

FIGS. 12, 13 and 14 are graphs showing the low gas pressure drop for thepacking of the invention at zero liquid flow (FIG. 12), and at two ratesof liquid flow (FIGS. 13 and 14); and

FIG. 15 is a graph showing the critical effect of average pore size onthe operation of the packing of the invention.

Referring now to FIGS. 1 to 4, the reference numeral 10 refers generallyto a packing body in the form of a small porous cylinder which may bee.g., 1" in diameter and 1 /2" long. The cylinder 10 is made up of amultitude of irregularly shaped cells, one of which is shown in greatlyenlarged, semi-idealized form in FIG. 4. Referring to FIG. 4, it may beseen that the cell is made up of a plurality of strands 11 which areinterconnected with one another at points 12 to form a 3-dimensionalnetwork. As may be seen, the intersecting strands form a multitude ofirregularly shaped pores through which the cells communicate with oneanother. For example, pore 13 is bounded by the strands which define theplane ABCDEA. Pore 14 is bounded by the strands defining plane FGHF.

In FIG. 4, only one cell is shown for clarity of illustration, but it isunderstood that the multitude of individual open cells making up thepacking body are each connected directly to other cells by strands suchas strands 11a, 11b, and He which radiate from the various points ofintersection of the strands to form additional pores making upadditional cells. The structure thus formed is accordingly completelyopen and pervious to fluid flow in all directions and characterized by ahigh free volume. The free volume (i.e., the total volume of the elementless that occupied by the interconnected strands) will be at least andpreferably at least and will often be of the order of to 99%.

In the packing body of the invention, it is important that theindividual pores vary substantially in size. This feature is illustratedin FIG. 3 showing a greatly enlarged view of a po tion of thecross-section shown in FIG. 2, where the pore size variation may beseen. As seen in that figure, the strands 1.1 intersecting at points 12form a wide range of pore sizes, ranging from relatively large pores asindicated by the shaded area designated 15, to small pores as indicatedby the shaded area 16. The variation in pore size is preferably suchthat the larger pores are at least about five times the cross-sectionalarea of the smallest pores. The relatively wide variation in pore sizecharacteristic of the bodies making up the packing of the invention isof critical importance for the following reasons. Where there is asubstantial variation in pore size, liquid return produced by coalescedliquid droplets will tend to flow along paths Where the fiber density ishighest, corresponding to the smaller pore sizes. This is a dynamicproperty, because if the liquid flow is stopped, the channels throughwhich liquid is preferentially flowing will rapidly drain, with littleor no liquid being retained by static capillary forces in the pores.This preferential flow through the maximum fiber density paths withinthe pellet (corresponding to the sites where the small pores areconcentrated) may be termed dynamic conductivity. An appreciable poresize variation confers the very necessary property of flexibility indynamic conductivity. Thus, at low liquid entrainment rates, whereliquid flow through the packing is correspondingly low, the smallerpores will be the principal conductors of liquid in the form of numerousrandom, tortuous, small-volume liquid streams flowing through the3-dimensional packing and bed structure. At such liquid flow levels, thefractional internal volume of each porous body remaining open andavailable for gas flow and disentrainment function is quite large.

As the liquid load increases, the flow channels either enlarge to occupythe next lower density fiber paths adjacent to the establish liquiddischarge streams (corresponding to sites where somewhat larger poresare located), or new streams are formed. The dynamic conductivity of theindividual porous bodies and the bed itself thus changes to .accommodatethe additional liquid load by reason of the pore size variation. The gasflow is still continuous, and the filtering or disentrainment action ofthe porous bodies still not occupied by liquid flow is enhanced becausethe actual gas velocity in the free voids increases and the dropletimpingement velocity goes up.

Ultimately, the liquid load may be increased to the point where thelowest fiber density volumes of the pellet (corresponding to sites wherethe largest pores are located) will be called into play to carry theflowing liquid. However, even at this point, where the individual bodiesmay be fully occupied by dynamic liquid flow, the interstitial voidsbet-ween the pellets (where the fiber density is zero) still serve tocarry the gas stream, and the bed does not flood.

Another advantageous result of the pore size variation is the easierdischarge of liquid from the bed of porous bodies than from a pad. Thepore size variation causes the returning liquid to form streams whosecross-section of flow is considerably less than that of the vesselcontaining the bed, as opposed to the nature of flow in a pad or bed ofuniform pore size material wherein the liquidreturn flow area wouldoccupy the whole of the fibrous material. Thus, at equal liquidentrainment loadings, the bed of nonuniform porous bodies would have ahigher equilibrium hydrostatic liquid head in the liquid flow channelsthan would a uniform porous material, thus allowing liquid dischargeagainst higher frictional gas resistances.

In addition to substantial pore size variation, the average size of thepores is also of critical importance for insuring that the porous bodieswill be noncapillary and thus nonflooding in nature. For most commonliquid systems, .an average pore size varying between a. maximum poresize providing an average of about 5 pores per lineal inch to a minimumpore size providing an average of about 50 pores per lineal inch, willresult in noncapillary, i.e., nonflooding, operation of the porous body.The preferred limits of average pore size for most common liquid systemsare from 10 to 40 pores per lineal inch. The term pores per lineal inchmeans the average number of pores encountered in any cross-section ofthe porous body per lineal inch in .a straight line in one plane.

With porous bodies having a substantial variation in pore size and anaverage pore size controlled within the limits specified above, it hasbeen found that a bed of packing made up of such bodies has a very highresistance to flooding due to the fact that the porous bodies remainopen and permeable both to gas and liquid flow as contrasted to acondition where liquid progressively accumulates within the porous bodyand is held there in a static condition by capillary action causing thevoids within the body to flood. In this latter condition, the liquidwithin the body is essentially quiescent and gas flow through the bodyis blocked. In the noncapillary, nonfiooding bodies of the invention,dynamic liquid flow occurs through the cross-section of the porous bodyin a series of irregular streams or rivulets as explained above.

The criticality of average pore size with respect to obtaining thedesired noncapillary, nonflooding characteristics has been demonstratedby a series of tests which will be described hereinafter.

:It is to be particularly noted that the pore structure of the packingbody of the invention is 3-dimensional rather than Z-dimensional innature in contrast to prior packings consisting of hollow shapes made upof thin screen elements. As is apparent from the drawings, thecross-sectional thickness of the bodies of the invention (thecrosssectional thickness of the cylindrical body of FIGS. 1 to 4 beingits diameter D) is large relative to the average pore diameter, beingpreferably at least 5 times as great.

While the 3-dimensional network making up the packing body need not beabsolutely rigid (i.e., capable of resisting deformation by compressiveforces), it should at least have sufficient rigidity to be substantiallyselfsupporting under conditions of use. This is, the packing bodiesshould not deform substantially under conditions of use so as toeliminate voids between the individual bodies.

While the method by which the packing body shown in FIGS. 1 to 4 may beprepared forms no essential part of the present invention, oneconvenient and economical method for producing such a structure is bythe foamblowing of plastics, metals and ceramics in such manner as toform an open-celled structure entirely pervious to fluid flow. Thus,open-celled polyurethane foams or other open-celled plastic, metal orceramic foams may be employed. The foams may be initially formed intothe individual packing bodies, or the foam may be prepared in bulk andthen cut into the desired packing bodies of the desired size. If thefoam, as originally produced, is not sufficiently rigid in character, itmay be rigidified by providing a thin coating of a rigidifying materialsuch as a rigid plastic coating, e. g., of a phenol formaldehyde resinor an epoxy resin. In such a coating operation, care must be taken notto close the pores so as to leave the rigidified structure completelypermeable to fluid flow.

FIGS. 5 to 8, inclusive, illustrate a packing body having a similar3-dimensional structure of interconnected open cells communicating withone another through a multitude of pores wherein the structure isobtained by a random interlacing of fibrous material such .as metalfibers, glass fibers, plastic fibers, or the like, the points ofintersection of the fibers being bonded to one another, while leavingthe spaces between the fibers open so as to provide the random porestructure desired. A typical open cell formed by this type ofconstruction is shown in FIG. 8 wherein reference numeral 17 indicatesthe individual fiber strands bounded at their points of intersection 18to form an open cell communicating with adjacent cells through poressuch as pores 19 and 20. Referring to FIG. 7 showing an enlargement ofthe cross section of FIG. 6, it may be seen that the pores are irregularin shape and vary in size over a relatively wide range, includingrelatively large pores indicated by 21 to small pores indicated by 22.While the packing body of FIGS. 5 to 8 may be made by any desiredmethod, one suitable procedure is to make up a randomly felted mat offibers which is then treated to bond the fibers at their points ofintersection such as by impregnation with a resin (taking care not toblock the pores interconnecting the cells) to form a suitably rigidstructure which may then be cut (e.g., by diepunching) into regularsmall shapes such as cylinders or pellets of polygonal cross-section. Inthe case of a metal fibrous material, such as steel fibers, the contactpoints may be fused by passing an electric current through.

The packing body should be small in size compared with the operatingvolume to be filled therewith. For the majority of applications, theaverage dimensions will range from about to 4", and more usually fromabout /2" to 3". As previously stated, its overall size and shape shouldbe such as to provide, when the bodies are dumped into a contact towerin random arrangement, a relatively large void volume between theindividual bodies in contrast to a tightly packed arrangement affordinglittle or no void volume. Preferably the inter-body void volume shouldbe at least 15% of the total volume occupied by the bed of bodies inrandom arrangement and may range up to about To this end, the shape andsize of the individual bodies is such that when randomly dumped to forma bed, there is predominantly point-to-point, or lineto-line contactbetween the bodies rather than planar contact. Preferred shapes for thepacking bodies making up the packing of the invention are cylindricalshapes such as shown in the drawings, thick-walled hollow cylinders,spheres, pellets of polygonal cross-section, or the like. The contactbodies are likewise preferably so shaped that the dimensions along anygiven axis do not vary greatly. Preferably, the longest dimension is notgreater than 3 times the shortest dimension, and most desirably, notmore than twice.

Reference is now made to FIG. 9 which shows a portion of a bed ofpacking bodies of the invention in the form of porous cylinders 10having a length slightly greater than their diameter. When such shapesare dumped at random in a contact tower to form a bed of packing, arelatively large volume of voids 23 results between the individualpacking bodies. With this arrangement, gas flow may occur through theinter-body voids 23 around the external surfaces of the packing bodies10, and in addition, gas flow may also occur through the interior of thepacking bodies themselves, by passing through the relatively large poresnot occupied by liquid streams.

A further desirable characteristic of the packing bodies of theinvention in contrast to the use of fibrous packing in the form ofsheets or wads, is the manner in which the multitude of externalsurfaces provided by the small individual packing bodies tends todeflect or refract liquid flow to provide a zigzag liquid flow patternand to distribute returning liquid streams uniformly over thecrosssectional area of the bed. This deflecting or retracting action ofthe contact elements is illustrated in FIG. 9 wherein the arrowsindicate direction of liquid flow. For example, a stream of liquidindicated by the arrow 24, on encountering the slightly inclined surfaceof packing body 10a at point 25, is deflected or refracted along line 26rather than continuing in a straight-line direction of flow. The randompacking arrangement shown with a multitude of surfaces at randominclination to one another, separated by inter-body voids, is requiredin order to obtain this effect.

As previously indicated, a highly advantageous feature of the packingelements of the invention is their low resistance to gas and liquid flowas indicated by low gas pressure drops and their high resistance toflooding even at high gas and liquid rates. Comparative pressure dropmeasurements were made for beds packed with /2 ceramic Raschig rings,/2" x /2" solid wooden cylinders, and /2" x /2 cylinders of open-porepolyurethane foam having a 3-dimensional network of interconnectedstrands forming a multitude of cells communicating with one anotherthrough open pores as illustrated in FIGS. 1 to 4. The average pore sizewas such that the material averaged 25 pores per lineal inch. The poresize ranged from a maximum of about 0.1 inch for the large pores to aminimum of about 0.01 inch for the small pores. These three materialswere tested in a packed bed 3" in diameter and 24" deep. The bed wasformed in each case by dumping the cylinders or Raschig rings into thecontact tower in random arrangement. Liquid was delivered into the topof the bed by a spray head which distributed the liquid over thepacking. Air was passed into the bed flowing upwardly countercurrent tothe liquid. Air pressure drop measurements were made at various liquidflow rates. The results of these tests are shown graphically in FIGS.12, 13, and 14.

FIG. 12 shows the pressure drops obtained with no liquid flow (packingsin dry condition) illustrating the considerably lower pressure drop forthe packing elements of the invention.

FIG. 13 shows the pressure drops obtained for the three types of packingat a liquid flow rate of L =lO0 pounds per hour per square foot ofpacking at varying gas velocities. As may be seen, the pressure drop forthe packing of the invention is not only very substantially lower thanthe pressure drops for other types of packing, but it will also be notedthat the rate of increase of pressure drop is constant over a wide rangeof gas velocities. Thus, the wood cylinders and Raschig rings show asteep rise in the pressure drop at gas velocities of the order of 300 to450 pounds per hour per square foot, indicating the onset of floodingconditions whereas the packing of the invention shows no such sharpincrease of pressure drop even at gas velocities up to 1000 pounds perhour per square foot, thus indicating a high resistance to flooding forthe packing of the invention.

FIG. 14 shows the results obtained for the three types of packing at aliquid rate of 5000 pounds per hour per square foot of packing atvarious gas rates. Here again, the same relative behavior is to benoted. The wood cylinders and Raschig rings show a steep rise inpressure drop at gas velocities between 200 and 400 pounds per hour persquare foot, indicating the onset of flooding, whereas the packing ofthe invention shows no tendency to produce a steep rise in pressure dropat much higher gas velocities, again indicating a strong resistance toflooding.

As stated previously, the average pore size of the bodies making up thepacking of the invention is a critical factor governing its floodingresistance and thus its operability as a noncapillary, nonfloodingdisentrainment medium. In a first series of tests to show the effect ofaverage pore size on the flooding resistance of the packing of theinvention, porous packing bodies were employed having 20, 30, 40 and 60p.p.l.i. (pores per lineal inch), respectively, having a structure asshown in FIGS. 1 to 4. The packing bodies were placed in a six-inchdiameter column between retaining screens to provide a bed about twelveinches in depth. These beds, in each case, were first loaded with waterby passing an air stream containing entrained water droplets upwardlythrough the bed. The bed was then allowed to drain by gravity for fiveminutes. Thereafter, an air stream was passed upwardly through the bedat a superficial velocity of 12 feet per second in one series of runs,and at a superficial velocity of 13.5 feet per second in a second seriesof runs. The pressure drop (AP) in feet of water per foot of packingdepth was measured for each pore size, and the results are showngraphically in FIG. 15 where Curve 60 shows the results at 13.5 feet persecond, and Curve 61 shows the results at 12 feet per second.

As can be seen in FIG. 15, the pressure drop for the 20, 30 and 40p.p.i. packings remained low although increasing somewhat withdecreasing pore size (i.e., with increasing numbers of pores per linealinch), but increased very sharply as the average pore size decreased tothat corresponding to 60 p.p.i. The very sharp increase in bedresistance at the 60 p.p.i. level indicated that it was staticallyflooded through capillary action (high liquid hold-up in the bedfollowing the five-minute drainage period) which was verified by visualobservation. As shown by Curves 60 and 61 in FIG. 15, the average poresize (as measured in pores per lineal inch-p.p.i.) has a critical effecton the operability of the packing of the invention even under a staticliquid load (i.e., loaded with liquid but no dynamic liquid flow) withan extremely sharp and unacceptable increase in the gas flow resistancebeginning at about p.p.i. average pore size.

The same critical eifect of average p.p.i. on operability was also foundin a second series of tests run under conditions of dynamic liquid flow.In this second series, a twelve-inch deep bed of the packing bodies wasplaced between retaining screens in a six-inch diameter column.Entrainment was generated by means of a splash plate (a perforated platethrough which air was passed while a heavy jet of water was directedagainst a solid central target section of the plate). The bottom of thebed of packing bodies was positioned about 17 inches above the splashplate. Quantitative measurements of the liquid entrainment load in theair stream at various air velocities were made at the level of thebottom of the bed. With this arrangement, the disentrainment efliciencyof porous packing bodies consisting of one-inch diameter cylindersconstructed according to FIGS. 1 to 4 and having 30, 40 and p.p.i.average pore size, respectively, was measured.

In runs with the 30 p.p.i. packing bodies, when the gas velocity andentrainment load were increased to a maximum superficial gas velocity ofabout 14 feet per second, giving a liquid entrainment load of over 300lbs. per hour per square foot of bed cross-section, a measureddisentrainment efliciency (i.e., percent of entrained liquid removed bythe bed) of 99.9% was obtained at a stable, constant pressure drop of4.50 inches of water over an extended period of time. Similar resultswere obtained with the bed consisting of 40 p.p.i. packing bodiesalthough moderate amounts of entrainment breakthrough occurred after aperiod of operation at the maximum gas velocity of 14 feet per secondcorresponding to entrainment loads of about 300 lbs. per hour per squarefoot.

When attempts were made, however, to repeat this test with a bed ofpacking bodies of 60 p.p.i. average pore size, the bed floodedimmediately and gave zero disentrainrnent efficiency at even the lowestliquid loadings. Attempts to increase the disentrainment efiiciency andavoid flooding, using the 60 p.p.i. packing bodies, by changing pelletdiameter to one-half inch and by reducing the depth of the pellet bed to9 inches similarly failed. No disentrainment was achieved and thepressure drop through the bed built up to the point that the bed itselfand the retaining screen holding the bed were blown out of the column.

In still further tests, the extraordinary capacity of the packings ofthe invention was demonstrated by placing a 15-inch deep bed of packingconsisting of one-half inch diameter pellets about 4 inch long, havingthe porous structure shown in FIGS. 1 to 4 and an average pore size of30 pores per lineal inch, in a 6-inch diameter column directly on thesplash plate described above so that the bed was subjected to the fullintensity of the extremely high entrainment load generated at the splashplate. In these tests, when the gas velocity and correspondingentrainment load Was gradually increased, the packing gave a measuredefficiency of over 99.9% entrainment removal at all air velocities up toa gas velocity of 8 feet per second and an entrainment load of over30,000 lbs. per hour per square foot of cross-section. There were nosigns of any instability or entrainment leakage through the bed up to avelocity of 8 feet per second and an entrainment load of over 30,000lbs. per hour per square foot. This value of entrainment load is vastlyin excess of the maximum load that can be handled by any priordisentrainment media such as pads or other types of packages.

The choice of optimum average pore size for the bodies making up thepacking of the invention within the limits of about 5 to 50, andpreferabley 10 to 40, pores per lineal inch will usually be most easilyaccomplished by empirical methods. The larger pore sizes generallyprovide somewhat lower pressure drop while the smaller pore sizes aregenerally more effective in removing smallsize, entrained liquiddroplets. The choice of optimum pore size within the above range willgenerally be a balance between these two considerations. Sometimes itmay be desirable to employ a bed having bodies of two or severaldifferent average pore sizes within the above range with the larger poresize packing in the bottom of the bed where the larger liquid dropletsare removed, and the smaller pore size packing in the upper portion ofthe bed to effect removal of the smaller droplets. Such an arrangementtends to distribute the liquid disentrainment load more evenlythroughout the bed while, at the same time, increasing the range ofparticle size of liquid droplets that are effectively removed by thebed.

The extremely high effectiveness of the packing of the invention forgas-liquid disentrainment, as illustrated in the foregoing examples,stems not only from the internal structure of the porous bodiesthemselves (pore size variation and average pore size range beingcritical factors as explained above), but also from the combination ofthis specific internal structure of the individual bodies and theinter-body voids (i.e., the voids 23 of FIG. 9). In contrast todemisting pads and the like substantially filling the cross-section ofthe tower, the packed bed of the invention permits deeper bedpenetration of the gas-borne liquid droplets since they are able to passup through the bed through the inter-body voids. Demisting pads on theother hand tend to remove the bulk of the entrained droplets in thefirst portion of the bed encountered, thus tending to cause liquidoverloading (with accompanying high back pressure and erratic flowcharacteristics) of the inlet portion of the pad. Transmittance throughthe interbody voids in accordance with the invention permits fullerutilization of the upper portions of the bed by more evenly distributingthe entrainment trapping load throughout the bed. Since practically allback flow or drainage of liquid occurs through the porous bodiesthemselves (and very little through the inter-body voids), the tendencyto overload with liquid is greatly reduced since the overall porosity ofthe bed (porosity of the bodies themselves plus inter-body voids) cannotdrop below the minimum contributed by the inter-body voids. These voidsaccordingly even at high liquid loading tend to remain free to transmitgas flow.

A further advantage flowing from the combination of the porous bodiesand the inter-body voids for disentrainment applications is theadditional disentrainment mechanism that is brought into play. Inporous-pad type demisters, there are two principal disentrainmentmechanisms, viz. impingement disentrainment which removes the largerparticles and diffusion which removes the smaller particles. To thesethe packed bed of the invention adds an eddying and swirling movement ofthe gases in the inter-body voids which tends to bring the entraineddroplets into contact with the fibers of the packing bodies bycentrifugal force.

Reference is now made to FIGS. 10 and 11 which show a typicalapplication for the packing of the invention as a disentrainment packingbetween the plates of a bubble-cap distillation tower. The tower shownsemi-diagramatically in FIG. 10 comprises a tower shell 30 having asection A equipped with bubble cap plates as shown in the partialvertical cross-section in FIG. 11.

Liquid feed is introduced into the tower through line 32 flowingdownwardly countercurrent to vapors produced by the boil-up of theliquid 33 in the bottom of the column. Vapors are taken off at the topof the column by line 34, sent through reflux condenser 35 feedingreflux back into the column through line 36. Overhead products are takenoff by line 37.

Liquid is withdrawn from the bottom of the column through line 38. Aportion of the bottoms is recirculated through reboiler 39 by line 40and reintroduced 10 into the column through line 41 while a portion istaken off by line 42 as bottoms product.

Referring to FIG. 11, it may be seen that section A of tower 30 isequipped with a plurality of plates 44, 45, 46, etc., carrying layers ofliquid 47. The plates are equipped with the usual risers 48 providedwith caps 49 to force the gas to flow into the layer of liquid on theplates. Each plate is equipped with a usual downcomer 50. The upper edge51 of the downcomers establishes the liquid level on the plates.

A limiting factor in bubble-cap columns is liquid droplet entrainmentfrom one plate to another with attendant loss in plate efliciency and adecrease in overall separation. The packing of the invention provides anexcellent entrainment trap when the space between the plates is filledwith the packing bodies as illustrated in FIG. 11. (For clarity ofillustration, only a portion of the space between the trays is shown asfilled with the packing of the invention, while in the embodiment shownit is intended that substantially the entire space be so filled.)

As shown in FIG. 11, the packing material fills substantially the entirespace between the plates including the space occupied by the liquidlayer. This simplifies installation of the packing (since no specialsupport is needed, the packing being supported directly on the surfaceof the plates). This arrangement has the further advantage that thepresence of the porous packing bodies immersed in the liquid on theplates does not interfere with the desired interaction of the gas andliquid on the tray, but will tend to limit longitudinal mixing of theliquid on the plate. Such mixing, particularly on large plates, tends toreduce plate efficiency.

For the type of application illustrated in FIGS. 10 and 11, thelightweight nature of the packing of the invention and the fact that itis readily handled :as small units is a great advantage. Theseproperties permit it to be installed between the plates by such means aspneumatic conveyors for blowing the packing bodies into place ratherthan by hand installation as in the case of continuoussheet demistingpacks currently used. For this type of application, a bonded, non-woven,fibrous material or a foamed plastic material having an open-cell,permeable structure, such as open-cell foamed polyurethane, areparticularly advantageous because of their very light weight. Suchmaterials, having a pore volume of about to 97% are extremely light inweight (of the order of one to two pounds per cubic foot in the drycondition) and thus could be installed in existing towers withoutentailing any modification of tower foundations or shell thicknessbecause of increased weight.

In the type of application illustrated in FIGS. 10 and 11 Where theporous bodies are employed as a packing between the plates of agas-liquid contact tower with a substantial liquid layer on each plate,the packing permits a substantial decrease in plate spacing since inconventional plate towers the major portion of the volume of the columnis purely entrained liquid droplet -disengage ment space. The type ofapplication illustrated in FIGS. 10 and 11 is not limited to bubble-captowers but may extend to any type of gas-liquid contact tower containinga plurality of superimposed plates carrying substantial liquid layerswith means, such as perforations or slots in the plate, to permit gas toflow upwardly through the liquid layer while liquid flows downwardlythrough downcomers or any other suitable means to provide for liquidflow downward from plate to plate.

When used as a packing between plates as in FIGS. 10 and 11, it is notnecessary that the packing fill the entire space between the platesalthough this will often be desirable. Thus, the packing may besupported above the liquid layer by a wide-mesh wire screen instead ofoccupying the space flooded by liquid as well as the space above theliquid. Not all of the space above the liquid need be occupied by thepacking; but for effective operation, the packing should extend acrossthe entire plate area so as to avoid by-passing.

The packing of the invention is highly useful in any type of gas-liquiddisentrainment operation where the function of the bed is to disentrainliquid droplets carried by a moving gas stream such as a demisting bedin knockout drums, a demister at the outlet of distillation towers,scrubbing towers or the like to disentrain liquid droplets carried inthe gaseous efiluent from such towers.

It is to be understood that the invention is not limited to the specificillustrative embodiments described and that many other modifications andembodiments within the general spirit of the invention are includedwithin its scope.

I claim:

1. A method for disentraining liquid droplets from a moving gas streamcomprising the step of passing the gas stream containing said liquiddroplets through a. packing made up of a plurality of porous, butnoncapillaiy, bodies of relatively small size compared with theoperating volume occupied by said packing, said bodies being randomlydisposed in said operating volume, and so shaped as to provide in suchrandom arrangement a relatively large void volume between said bodies,said bodies being made up of a multitude of irregularly shaped opencells of varying size formed by a multitude of interconnected strandsproducing a 3-dimensional network throughout said bodies substantiallydevoid of walls, said cells communicating with one another throughirregularly shaped pores of varying size bounded by said interconnectingstrands whereby said bodies are open to fluid flow through said 3-dimensional network of open, substantially wall-less cells in alldirections, the average size of said pores being such as to provide anaverage of from about 5 to about 50 pores per lineal inch withsubstantial variation in size between the larger and smaller pores, thecross-sectional thickness of said bodies being large relative to theaverage size of said pores and the material forming said pores beingsuificiently rigid to resist substantial deformation under the loadimposed thereon under operating conditions.

2. A method in accordance with claim 1 in which the packing employed hasan average pore size of from about to about 40 pores per lineal inch.

3. In the operation of a gas-liquid contact device comprising aplurality of superimposed, horizontal plates carrying in operationliquid layers with a vapor space above said liquid layers with meanspermitting liquid to flow downwardly from plate to plate, and with meanspermitting gas to flow upwardly from plate to plate while passingthrough said liquid layers on said plates, whereby said gas streamentrains substantial quantities of liquid droplets and tends to carrythis entrainment upwardly from plate to plate, the method fordisentraining said liquid droplets from said gas stream which comprisesproviding the vapor space between said plates with a packing as definedin claim 1 extending across said vapor space in the path of gastraveling therethrough.

4. A method in accordance with claim 3 in which said packing issupported directly on said plates and substantially fills the entirespace between said plates, including that occupied by said liquidlayers.

5. A method in accordance with claim 3 in which said packing has anaverage pore size of from about 10 to about 40 pores per lineal inch.

References Cited UNITED STATES PATENTS 1,654,925 1/1928 Drager 261-94 X2,095,460 10/ 1937 Swords -523 X 2,921,776 1/ 1960 Keeping 261-942,961,710 11/1960 Stark. 3,190,057 6/1965 Sinex. 3,227,429 1/1966 Renzi261-112 3,266,787 8/1966 Eckert 261-94 3,293,174 12/1966 Robjohns 261-98X FOREIGN PATENTS 171,772 11/1921 Great Britain.

858,127 l/ 1961 Great Britain.

931,853 6/1963 Great Britain.

OTHER REFERENCES Support Plates and Distributors for Packed Towers, TheUS. Stoneware Co., Akron, Ohio. Bulletin TA-30, copyright 1957, pp. 4and 5 relied on.

HARRY B. THORNTON, Primary Examiner.

TIM R. MILES, Assistant Examiner.

